When viewing a scene, the human eye automatically factors out the color of an illuminating light source so that colors of objects in the scene appear as the colors would appear under a white light source. This process is known as “chromatic adaptation” or “white balancing.” For digital cameras, if white balancing is not performed well then captured images may appear to have a color bias or some colors in a captured image may not appear natural compared to other colors in the captured image. Color bias due to poor white balancing is one of the most common problems cited by photographers when assessing the quality of images captured using digital cameras.
Many conventional cameras provide an automatic white balance mode, referred to as “single image automatic white balancing”, which automatically adjusts white balance using one of many conventional methods. One such conventional white balancing method is known as “gray-world”. The gray-world method works well in many environments but often performs badly in low or unusual light conditions.
For photographic conditions in which conventional white balancing methods do not work, it is common for cameras to provide a set of special modes. These special modes are based on one or more models, such as incandescent light, fluorescent light, full sun, and shade, for example. However, even special modes sometimes give poor results if the ambient light in a scene does not fit the model well. For example, special modes often give poor results in photographic conditions such as light from low wattage incandescent lamps, dusk and dawn, indoor settings where the light is colored, or where there may be multiple illuminant types of light present.
Conventional cameras often include a sensor known as a White Balance (WB) Sensor mounted on the camera to detect the color of the light illuminating a scene to be captured. Such sensors aid a photographer or the camera (i.e., if the camera is working in an automatic mode), to select a special mode depending on the light illuminating the scene to be captured. However, such sensors only describe light striking the sensor and this may not correspond to the light illuminating the scene to be captured.
Some conventional methods for adjusting white balance require a user to capture an image of a white object such as a sheet of paper. These methods tend to be problematic as their result is dependent on being able to photograph a white surface under the same illuminant as a subject to be photographed.
The special modes described above are also prone to causing user error especially in a situation where a mode is selected by the user based on particular photographic conditions and then the user forgets to change the mode when moving to a different location where the conditions may be different. The use of a flash device may reduce white balance problems in many such situations. However, the use of a flash may also adversely affect the appearance of a captured image. This leaves a significant class of photographic conditions where color balancing remains problematic.
The use of fill flash in photography is well known. Fill flash is a light source providing supplementary illuminant to the normal flash illuminant provided by most conventional cameras. Fill flash is often used to shine a little supplementary light in certain regions of a scene being captured or to provide extra light for the entire scene.
The amount of flash illuminant used when capturing an image is difficult to control. In particular, the contribution of the fill flash to a scene can not easily be adjusted at a post-capture stage to obtain an optimum illumination level for the scene to be captured.
In a situation with mixed lighting, it may be difficult to control and change the color of flash illuminant used during an image capture process in order to match the color of the ambient light of the scene being captured. Similarly, at the post-capture stage it may be difficult to isolate and change the color of the fill flash component of the flash illuminant when adjusting color balance.
Some photographers have suggested a post capture flash tuning process as an application of recent advances in High Dynamic range image rendering. However, a problem with such an approach is that rendering is considered after application of the flash illuminant. As such, there is no control over the relative proportions of flash and ambient illuminant used to light the scene being captured.
The relative proportions of flash and ambient illuminant used to light the scene being captured is important when determining the optimum illumination level for the captured image of the scene. For example, FIG. 11A shows an image 1110 of a scene captured without any flash illuminant. The image 1110 includes three subjects 1130, 1140 and 1150. In the image 1110, the foreground subject 1130 is in shadow and is therefore poorly exposed (i.e., too little light from the foreground subject 1130 was captured) compared to the subject 1140 at mid distance and the subject 1150 at a longer distance in the image 1130. In contrast, FIG. 11B shows a second image 1120 of the same scene as FIG. 11A, captured with a supplementary flash illuminant (i.e., fill flash). One problem with the image 1120 of FIG. 11B is that the foreground subject 1130 may be relatively overexposed (i.e., too much light from the subject 1130 is captured) relative to the mid-distant subject 1140 and the distant subject 1150. Another problem with the image 1120 of FIG. 11B captured with fill flash is that the foreground subject 1130 is illuminated by flash illuminant whereas the other subjects 1150 and 1140 are illuminated by a mix of ambient and flash illuminant. If the color of the flash illuminant is different to the color of the ambient illuminant then the white balance for at least some of the subjects 1130, 1140 and 1150 may often appear incorrect.
None of the conventional methods of generating digital images allow post-capture control over the flash illuminant and ambient illuminant in a captured image of a scene while addressing white balance problems.
Thus, a need clearly exists for an improved method of generating a digital image of a scene.